There are numerous types of equipment and processes available for printing signs or other products with graphic designs, characters or other graphic images. Typically, a sheet or web of the material upon which the graphic images are printed (the "print sheet") is passed between a platen and a printing head, and the graphic images are transferred or formed on the print sheet by the printing head. In some devices, the print sheet is thermally-sensitive, and the printing head includes heating elements that are selectively activated to form graphic images on the thermally-sensitive sheet. In other devices, a donor web or foil bearing printing ink is passed between the printing head and the print sheet. The printing head typically includes heating elements which are selectively activated to transfer ink from the donor web onto the print sheet to form graphic images.
With the development of computer-aided-design (CAD) systems, in the fields of printing and signmaking, for example, single or multi-colored images can be created and stored in a data base and transferred onto a print sheet to produce customized signs or other printed matter. In the signmaking field, graphic images are printed onto print sheets made of vinyl or other plastics secured by a pressure-sensitive adhesive on a releasable backing material. Once the graphic images are transferred to, or formed on a vinyl print sheet, the print sheet is cut, typically with an automatic cutter, along the outline(s) of the graphic images, and the cut pieces are then attached to a sign board by means of the pressure-sensitive adhesive after removal of the releasable backing material. Sophisticated multi-colored and/or enhanced graphic images, including two-dimensional images that appear three-dimensional, can be readily created and transferred onto a print sheet to form such customized signs or other printed matter with relative ease and rapidity.
The collection of dust particles or other debris on the print sheet can significantly degrade the print quality of the graphic images, particularly in more sophisticated systems, which print graphic images with a relatively fine degree of resolution. Dust particles or other minute debris can cause fuzziness, blurring or other aberrations within the printed graphic images which can noticeably degrade the print quality of even the most sophisticated printing or signmaking systems. In systems employing sheet material with punched holes along the edges of the sheet for feeding the sheets through the printing apparatus, loose particles or fibers of the sheet material frequently remain in and around the punched holes from the hole-punching operation. This minute debris also tends to drift or collect on the printing surface of the sheet material, and in turn degrades the print quality of the printing apparatus.
In printing apparatus employing thermal-activated printing heads, such as the type described above in which the donor web or foil bearing printing ink is passed between the printing head and the print sheet, dust particles or other minute debris can also collect on the donor web and affect the thermal-transfer properties of the web. This too can cause fuzziness, blurring or other aberrations within the printed graphic images which can noticeably degrade print quality.
Static electricity can increase the quantity of dust particles or other debris collected within printing apparatus. Static charges on the print sheet or on the donor web cause additional dust particles to collect on the print sheet or donor web, respectively, and further degrade the print quality of the system.
It is an object of the present invention to overcome the drawbacks and disadvantages of prior printing apparatus due to the collection of dust particles or liquids, or other debris in the apparatus and on the print sheets or donor webs in such apparatus.